Merge pull request #348 from gitter-badger/gitter-badge

Add a Gitter chat badge to README.md

README.md

 # RT-Thread #
+[![Gitter](https://badges.gitter.im/Join Chat.svg)](https://gitter.im/RT-Thread/rt-thread?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
 
 [![Build Status](https://travis-ci.org/RT-Thread/rt-thread.png)](https://travis-ci.org/RT-Thread/rt-thread)
 [![gitter](https://badges.gitter.im/RT-Thread/rt-thread.png)](https://gitter.im/RT-Thread/rt-thread)